This data has to do with the new iCloud storage APIs in iOS 8 and Yosemite as well as with the upcoming iCloud Drive online storage feature. It’s best to safely store anything of importance before tomorrow’s wipe:

CloudKit Note: All public CloudKit databases are scheduled to be emptied on Tuesday, July 22nd.

Update: Apple has emailed developers about the wipe, noting that iCloud Drive, Photos, and other iCloud-related products besides CloudKit storage will not be wiped:

Dear Developer, Starting July 22, 2014, CloudKit will require iOS 8 beta 4 or OS X Yosemite Developer Preview 4. To ensure uninterrupted development of your CloudKit enabled apps, download and upgrade to these versions today. We will perform a server-side data wipe on all CloudKit public databases on July 22. If you are using iCloud containers with public databases, consider creating a copy of your data so that you can re-upload once the data wipe is complete. iCloud Drive, iCloud Photo Library, and Mail Drop will not be impacted by the targeted data wipe. If you have any questions, visit the Apple Developer Forums. Best regards, Apple Developer Technical Support